An apparatus for carrying out tomography is typically arranged in such a manner that a radiation source and a multichannel radiation detector is rotated about a subject to be examined, the radiation source disposed so as to generate a fan beam of radiant rays such as X-rays, the multichannel radiation detector disposed in opposition to the radiation source with the subject being interposed therebetween, the fan beam of radiant rays being applied in the direction parallel with a chosen section of the subject, the chosen section of the subject being measured in a plurality of directions parallel therewith so as to obtain data representative of the distribution of the intensity of transmitted radiant beams, and an image of the section of the subject being thereby/reconstructed on the basis of the data. In such an apparatus, in general, in order to enhance the resolution of the thus-reproduced image and/or reduce the occurrence of an artifact derived from aliasing or the like, the radiation detector is disposed so that a radiant ray passing through the axis of the rotation may be made incident upon a channel of the radiation detector at a point offset by an amount less than half of one channel width from the center of the channel (hereinafter referred to as "offset arrangement"). An image representing the section of the subject is reconstructed on the basis of the data obtained from the thus-arranged radiation detector. Typically, a quarter of channel width is selected as such an offset amount.
In this case, the acquired data has an array corresponding to the form of the fan beam of radiant rays. The/reconstruction of the sectional image of the subject may be performed directly through this data array or after the data array has been converted to another data array corresponding to the form of a parallel beam of radiant rays.
Also, the specification of Japanese Patent Laid-open No. 206729/1983 describes a further improvement in the arrangement of the type in which an image is/reconstructed after the array of acquired data has been converted to a data array corresponding to a parallel beam of radiant rays. In the arrangement described in this specification, a processing such as convolution, filtering or the like is performed each time data corresponding to a parallel beam in a different direction is obtained. Subsequently, after a set of the processed data relating to the parallel beams in all directions has been prepared, pieces of data provided by beams which are shifted by a half channel width and the directions of which are opposite to each other are alternately arranged in an inserted manner. Thus, a processing such as back projection is effected on the basis of a set of the inserted data.
In a case where the/reconstruction of an image is to be performed directly through the data having an array corresponding to the fan beam of radiant rays, the effect of offset arrangement of the radiation detector is not necessarily be fully utilized from the viewpoints of an improvement in the resolution of the/reconstructed image and/or a reduction in the occurrence of an artifact derived from aliasing or the like. In contrast, in a case where the reproduction of an image is to be performed from an ideal parallel beam of radiant rays, the effect of offset arrangement of the radiation detector is fully realized. However, if a fan beam of radiant rays is to be converted to a parallel beam of radiant rays, a further arithmetic operation is required to effect conversion of the data array. In addition, this conversion involves a lowering of the quality of a/reconstructed image, a deterioration of the resolution or the occurrence of an artifact. The apparatus described in the aforesaid specification laid open to a public inspection has a further problem in that it provides no substantial effect of reducing the occurrence of an artifact.